U.S. patent application number 16/136552 was filed with the patent office on 2019-01-17 for procedure for setting laser and heater power in hamr device.
The applicant listed for this patent is Seagate Technology LLC. Invention is credited to Kai Chiu Cheung, Wai Yuen Lai, Tim Rausch, Tatsuya Shimizu.
Application Number | 20190019531 16/136552 |
Document ID | / |
Family ID | 57320743 |
Filed Date | 2019-01-17 |
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United States Patent
Application |
20190019531 |
Kind Code |
A1 |
Rausch; Tim ; et
al. |
January 17, 2019 |
PROCEDURE FOR SETTING LASER AND HEATER POWER IN HAMR DEVICE
Abstract
A heater power of a heat-assisted magnetic recording head is set
based on an initial head-medium clearance estimate in response to
the heater power. For a plurality of iterations, an optimum laser
power of the recording head is determined and a heater power is set
for a next iteration that results in an optimum heater power for
the optimum laser power. If differences in the heater and laser
powers between two subsequent iterations are below thresholds, the
iterations are stopped and the optimum heater power and the optimum
laser power for one of the subsequent iterations are used as an
operational heater power and an operational laser power.
Inventors: |
Rausch; Tim; (Farmington,
MN) ; Shimizu; Tatsuya; (Tokyo, JP) ; Lai; Wai
Yuen; (Tokyo, JP) ; Cheung; Kai Chiu; (Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Seagate Technology LLC |
Cupertino |
CA |
US |
|
|
Family ID: |
57320743 |
Appl. No.: |
16/136552 |
Filed: |
September 20, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14716148 |
May 19, 2015 |
10090011 |
|
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16136552 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11B 2005/0021 20130101;
G11B 5/6088 20130101; G11B 5/607 20130101 |
International
Class: |
G11B 5/60 20060101
G11B005/60 |
Claims
1. A method comprising: setting a heater power of a heat-assisted
magnetic recording head based on an initial head-medium clearance
estimate in response to the heater power; for a plurality of
iterations: determining an optimum laser power of the recording
head based on writing data to at least one track of a recording
medium at the heater power; applying an additional heater power to
approach or cause a head-medium contact at the optimum laser power;
and based on the value of the additional heater power, setting the
heater power for a next iteration that results in an optimum heater
power for the optimum laser power; wherein, if a first difference
in the heater power between two subsequent iterations is below a
first threshold and a second difference in the optimum laser power
between the two subsequent iterations is below a second threshold,
stopping the iterations and using the optimum heater power and the
optimum laser power for one of the two subsequent iterations as an
operational heater power and an operational laser power for the
heat-assisted magnetic recording head.
2. The method of claim 1, wherein setting the heater power for the
next iteration comprises backing off from the additional heater
power by a predetermined amount known to induce a desired negative
displacement.
3. The method of claim 1, wherein setting the heater power for the
next iteration comprises gradually reducing the additional heater
power while actively measuring clearance until a desired clearance
is reached.
4. The method of claim 1, wherein the optimum laser power is
determined based on a signal-to-noise ratio of the at least one
track.
5. The method of claim 1, wherein the optimum laser power is
determined based on a bit-error rate of one or more tracks.
6. The method of claim 1, wherein the optimum laser power is
determined based on a servo error rate of the at least one
track.
7. The method of claim 1, wherein the optimum laser power is
determined based on a servo variable gain amplifier gain while
reading back the at least one track.
8. The method of claim 1, wherein the optimum laser power is
determined based on at least one of a width or amplitude of the at
least one track.
9. The method of claim 1, wherein the optimum laser power is
determined based on an amplitude of an overwrite signal.
10. The method of claim 1, wherein the method is performed during
operational use of the heat-assisted magnetic recording head.
11. An apparatus comprising: interface circuitry operable to
communicate with a heat-assisted magnetic recording head; and a
controller coupled to the interface circuitry and configured to
perform, via the interface circuitry, a procedure that involves:
setting a heater power of a heat-assisted magnetic recording head
based on an initial head-medium clearance estimate in response to
the heater power; for a plurality of iterations: determining an
optimum laser power of the recording head based on writing data to
at least one track of a recording medium at the heater power;
applying an additional heater power to approach or cause a
head-medium contact at the optimum laser power; and based on the
value of the additional heater power, setting the heater power for
a next iteration that results in an optimum heater power for the
optimum laser power; wherein, if a first difference in the heater
power between two subsequent iterations is below a first threshold
and a second difference in the optimum laser power between the two
subsequent iterations is below a second threshold, stopping the
iterations and using the optimum heater power and the optimum laser
power for one of the two subsequent iterations as an operational
heater power and an operational laser power for the heat-assisted
magnetic recording head.
12. The apparatus of claim 11, wherein setting the heater power for
the next iteration comprises backing off from the additional heater
power by a predetermined amount known to induce a desired negative
displacement.
13. The apparatus of claim 11, wherein setting the heater power for
the next iteration comprises gradually reducing the additional
heater power while actively measuring clearance until a desired
clearance is reached.
14. The apparatus of claim 11, wherein the optimum laser power is
determined based on a signal-to-noise ratio of the at least one
track.
15. The apparatus of claim 11, wherein the optimum laser power is
determined based on a bit-error rate of one or more tracks.
16. The apparatus of claim 11, wherein the optimum laser power is
determined based on a servo error rate of the at least one
track.
17. The apparatus of claim 11, wherein the optimum laser power is
determined based on a servo variable gain amplifier gain while
reading back the at least one track.
18. The apparatus of claim 11, wherein the optimum laser power is
determined based on at least one of a width or amplitude of the at
least one track.
19. The apparatus of claim 11, wherein the optimum laser power is
determined based on an amplitude of an overwrite signal.
20. The apparatus of claim 11, wherein the procedure is performed
during operational use of the apparatus.
Description
RELATED PATENT DOCUMENTS
[0001] This application is a continuation of U.S. application Ser.
No. 14/716,148 filed on May 19, 2015, which is incorporated herein
by reference in its entirety.
SUMMARY
[0002] The present disclosure is directed to setting of laser and
heater power in a heat-assisted magnetic recording device. In one
embodiment, a method involves setting a heater power of a
heat-assisted magnetic recording head based on an initial
head-medium clearance estimate in response to the heater power. For
a plurality of iterations, an optimum laser power of the recording
head is determined based on writing data to at least one track of a
recording medium at the heater power. For each iteration, an
additional heater power is applied to approach or cause a
head-medium contact at the optimum laser power and, based on the
value of the additional heater power, the heater power is set for a
next iteration that results in an optimum heater power for the
optimum laser power. If a first difference in the heater power
between two subsequent iterations is below a first threshold and a
second difference in the optimum laser power between the two
subsequent iterations is below a second threshold, the iterations
are stopped and the heater power and the optimum laser power for
one of the two subsequent iterations are used as an operational
heater power and an operational laser power for the heat-assisted
magnetic recording head.
[0003] These and other features and aspects of various embodiments
may be understood in view of the following detailed discussion and
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] The discussion below makes reference to the following
figures, wherein the same reference number may be used to identify
the similar/same component in multiple figures.
[0005] FIG. 1 is a block diagram of a hard drive slider and media
arrangement according to an example embodiment;
[0006] FIG. 2 is a cross-sectional view of a read/write head
according to an example embodiment;
[0007] FIG. 3 is a flowchart of a procedure according to an example
embodiment;
[0008] FIG. 4 is a graph showing results of a calibration procedure
according to an example embodiment;
[0009] FIG. 5 is a block diagram of a system and apparatus
according to an example embodiment; and
[0010] FIG. 6 is a flowchart of a method according to an example
embodiment.
DETAILED DESCRIPTION
[0011] The present disclosure generally relates to detection and
control of head-media spacing in data storage devices and the
setting of the operational laser power in a heat assisted magnetic
recording device (HAMR). The detection of head-to-media spacing
becomes more challenging in what are referred to as heat-assisted
magnetic recording devices. This technology, also referred to as
energy-assisted magnetic recording (EAMR), thermally-assisted
magnetic recording (TAMR), and thermally-assisted recording (TAR),
uses an energy source such as a laser to heat a small spot on a
magnetic disk during recording. The heat lowers magnetic coercivity
at the spot, allowing a write transducer to change magnetic
orientation. Due to the relatively high coercivity of the medium
after cooling, the data is less susceptible to paramagnetic effects
that can lead to data errors.
[0012] Generally, recording heads may utilize heaters for fine
control of head-to media spacing. The heaters heat a portion of the
recording head that faces the recording medium. The heating causes
a local protrusion due to thermal expansion of the material.
Thermal protrusion can be finely controlled to maintain a desired
clearance between read/write transducers and the recording medium.
As will be explained in greater detail below, the introduction of a
HAMR energy source to the read/write head can complicate the
control of head-to-media spacing. Further, while conventional
read/write heads may be allowed to contact the recording medium
under some conditions, a HAMR device may be damaged if such contact
occurs while recording. This can make the estimation and control of
head-to-media spacing more difficult in a HAMR recording head.
[0013] In reference now to FIG. 1, a block diagram shows a side
view of a read/write head 102 according to an example embodiment.
The read/write head 102 may be used in a magnetic data storage
device, e.g., hard drive. The read/write head 102 may also be
referred to herein as a slider, read head, recording head, etc. The
read/write head 102 is coupled to an arm 104 by way of a suspension
106 that allows some relative motion between the read/write head
102 and arm 104. The read/write head 102 includes read/write
transducers 108 at a trailing edge that are held proximate to a
surface 110 of a magnetic recording medium 111, e.g., magnetic
disk. When the read/write head 102 is located over surface 110 of
recording medium 111, a flying height 112 is maintained between the
read/write head 102 and the surface 110 by a downward force of arm
104. This downward force is counterbalanced by an air cushion that
exists between the surface 110 and an air bearing surface (ABS) 103
(also referred to herein as a "media-facing surface") of the
read/write head 102 when the recording medium 111 is rotating.
[0014] It is desirable to maintain a predetermined slider flying
height 112 over a range of disk rotational speeds during both
reading and writing operations to ensure consistent performance.
Region 114 is a "close point" of the read/write head 102, which is
generally understood to be the closest point of contact between the
read/write transducers 108 and the magnetic recording medium 111,
and generally defines the head-to-media spacing 113. To account for
both static and dynamic variations that may affect slider flying
height 112, the read/write head 102 may be configured such that a
region 114 of the read/write head 102 can be configurably adjusted
during operation in order to finely adjust the head-to-media
spacing 113. This is shown in FIG. 1 by dotted line that represents
a change in geometry of the region 114. In this example, the
geometry change may be induced, in whole or in part, by an increase
or decrease in temperature of the region 114.
[0015] To provide dynamic control of head-to-media spacing 113 via
heat, the read/write head 102 may include (or otherwise be
thermally coupled to) one or more heating elements 116. The heating
element 116 (e.g., resistance heater) may be provided with
selectable amounts of power by a controller 118. An increase or
decrease in current causes and increase or decrease in the
temperature of the heating element 116, which results in expansion
or contraction at the media-facing surface 103.
[0016] In addition to controlling the heating element 116, the
controller 118 includes logic circuitry for controlling other
functions of a data storage apparatus. The data storage apparatus
includes at least the read/write head 102 and recording medium 111,
and may include other components not shown, such as spindle motor,
arm actuator, power supplies, etc. The controller 118 may include
or be coupled to interface circuitry 119 such as preamplifiers,
buffers, filters, digital-to-analog converters, analog-to-digital
converters, etc., that facilitate electrically coupling the logic
of the controller 118 to the analog signals used by the read/write
head 102 and other components not shown.
[0017] Other elements of the read/write head 102 may also provide
heat besides or in addition to the heating element 116. For
example, a write coil of the read/write transducers 108 may
generate sufficient heat to cause configurable deformation of
region 114. This deformation will only occur when the coil is
energized, e.g., when data is being written. Further, the
illustrated read/write head 102 is configured as a HAMR recording
head, which includes additional components that generate heat near
the read/write transducer 108. These components include laser 120
(or other energy source) and waveguide 122. The waveguide 122
delivers light from the laser 120 to components near the read/write
transducers 108. These components are shown in greater detail in
FIG. 2.
[0018] In FIG. 2, a block diagram illustrates a cross-sectional
view of the HAMR read/write head 102 according to an example
embodiment. The waveguide 122 receives electromagnetic energy 200
from the energy source, the waveguide 122 coupling the energy to a
near-field transducer (NFT) 202. The NFT 202 is made of a metal
(e.g., gold, silver, copper, etc.) that achieves surface plasmonic
resonance in response to the applied energy 200. The NFT 202 shapes
and transmits the energy to create a small hotspot 204 on the
surface 110 of medium 111. A magnetic write pole 206 causes changes
in magnetic flux near the media-facing surface 103 in response to
an applied current. Flux from the write pole 206 changes a magnetic
orientation of the hotspot 204 as it moves past the write pole 206
in the downtrack direction (z-direction).
[0019] The energy 200 applied to the near-field transducer 202 to
create the hotspot 204 can cause a significant temperature rise in
local region. The near-field transducer 202 may include a heat sink
208 that draws away some heat, e.g., to the write pole 206 or other
nearby heat-conductive component. Nonetheless, the temperature
increase near the near-field transducer 202 can be significant,
leading to local protrusion in the region of the write pole 206 and
near-field transducer 202.
[0020] From head to head, there is may be significant variation in
passive fly height, optical efficiency of the NFT and light path,
and other variations that may affect dynamic fly height response.
In such a case, the amount of protrusion from head to head due to
thermal effects and the amount of laser power needed to optimally
record will be different from head to head. In FIG. 3, a flowchart
shows an example of a method for setting the laser power that
corrects for these variations and ensures that a fixed clearance is
maintained at the optimal laser power.
[0021] The method represented in FIG. 3 may be performed during
qualification of a HAMR device, e.g., in the factory after
manufacture. The method may also be performed post-manufacture,
e.g., as part of a regular recalibration process, manually or
automatically initiated to mitigate performance issues, etc. While
the following procedure describes the setting and determination of
laser and heater power, this may also encompass indirect estimates
of power. For example, a controller may set power levels by
changing one or both of a current and a voltage applied to the
heater and laser. Generally, the values may be discrete values
input to a digital-to-analog converter (DAC), which converts the
discrete values to an analog voltage or current.
[0022] The procedure begins by initializing 300 a counting variable
N used in subsequent iterations. An initial estimate of correct
reader heater power HP.sub.N is made at block 302, and a laser
power value LP.sub.N is initialized to zero. The initial heater
power value HP.sub.N may be specific to certain conditions, such as
a zone/region of the recording medium being tested, ambient
temperature, rotation speed of the medium, etc. Both HP.sub.N and
LP.sub.N will be determined during an iteration, and compared with
corresponding HP.sub.N+1 and LP.sub.N+1 in subsequent iterations.
As such, the procedure will involve storage of heater and power
values from at least one previous iteration, and values for more
iterations may also be stored, e.g., to calculate measures such as
slope that indicate whether convergence is achievable.
[0023] At block 306, a series of data are recorded by sweeping
through a laser power value (e.g., LP.sub.0 to LP.sub.M) and then
reading back the data that was recorded at the different powers.
The heater and laser power values may be discrete values input to a
DAC, and so may sweeping through the laser power may involve
incrementing a discrete DAC input value. In one embodiment, the
laser power may be swept while recording a single track. In another
embodiment, three tracks are written and the middle track is used
as the reference track. With this method, the effects of adjacent
track interference can be accounted for. In another embodiment, a
whole track (or multiple tracks) can be written at one laser power
level, read back, and then the process repeated at a different
laser power levels over the same track(s) or equivalent tracks. As
with the heater power, the range of laser power levels used in the
sweep may be dependent on zone, temperature, and other factors.
[0024] At block 308, an optimal recording laser power is found.
This optimal recording laser power may be based on one or more
measurements made while reading back the recorded data, such as any
combination of signal-to-noise ratio (SNR), bit error rate (BER),
track average amplitude, track width, and overwrite performance.
For example, if SNR is used, the optimum laser power may correspond
to any combination of a peak SNR, target change in slope of SNR,
SNR for tracks written at increasingly smaller pitch (e.g.,
squeezed SNR), etc. If BER is used, the BER may be measured for an
isolated track and/or over multiple tracks. In the latter case, the
laser power can be chosen that corresponds to minimum BER of a main
track with two adjacent tracks written at a fixed pitch to simulate
drive operational conditions.
[0025] In some cases, neither SNR nor BER may be directly
available, and so other measurements may be made and analyzed at
block 308. For example, track average amplitude or track width may
be used. If track width is used, laser power may be increased until
a desired track width is achieved. Track width can be measured by
scanning the reader over a written track, e.g., reading the track
at various crosstrack offsets either side of the track center. The
width may be defined, for example, as the 50% amplitude point on
both sides of the track centerline. In another example where
neither SNR nor BER are directly available, servo error rate may be
used instead to determine optimal recording power. Servo error rate
is the error rate of the sync mark in tracks which are stitched
together during the writing process. This may be considered a
single-sided squeeze metric.
[0026] In some systems, like servo copy in a drive, neither SNR nor
BER are available. With servo copy, a seed pattern is written by a
specialized manufacturing machine. The drive then uses this seed
pattern to fill in the missing servo patterns needed by the drive.
In this case, the signal from the servo variable gain amplifier
(VGA) can be used to determine optimal recording power. When a
servo sync signal is written between the servo marks and user data
sectors, the signal is first adjusted using the servo automatic
gain control plant. A signal which is written weakly requires more
gain from the VGA. As laser power is increased, less gain is
required. The VGA gain can be used to pick the optimal laser power
at block 308, e.g., laser power that corresponds to a minimum value
of VGA gain.
[0027] In other embodiments, an overwrite signal can be used as a
metric for determining an optimal laser power. Overwrite generally
involves writing a tone at a first frequency and then immediately
over write that data at a second, different frequency. When the
signal is read it back, it is analyzed to determine components of
each frequency, and in particular how much of the first frequency
can still be read. The quality of the recording is inversely
related to how much of the first frequency is still present, e.g.,
the amplitude of the first frequency component. It will be
understood that any available measures described herein to
determine optimum laser power may be used in combination, e.g.,
each being a weighted contribution to an overall score, the optimum
laser power being the one with the best score.
[0028] At block 310, the laser is set to the optimum laser power,
which is designated as LP.sub.N+1 to differentiate it from either
the initial LP.sub.0 or LP.sub.N if this is not the first iteration
(N>0). Also at this block 310, heater is increased until
head-to-medium contact is detected, or until a suitably small
clearance is detected just before contact. Contact can be detected
by using an acoustic emission (AE) detector, which detects
vibrations resulting from the contact. Other contact and/or
clearance detection schemes may be used, such analyzing a thermal
profile detected by a thermal sensor located near the media-facing
surface of the read/write head. In another embodiment, contact
and/or clearance can be detected based on position error sensor
(PES) signals. For example, when the head contacts the medium, it
may skip to the left or right, resulting in a jump in the PES
signal. This block 310 may be performed in every zone or a subset
of zones, and so a contactless detection process may be preferred
to minimize damage to the read/write head.
[0029] At block 312, heater power is backed off by a particular
amount to maintain the target clearance using the optimum laser
power LP.sub.N+1 for this iteration. The heater power so obtained
is considered an optimum heater power for the iteration and is
designated LP.sub.N+1 for this iteration. For example, the optimum
heater power LP.sub.N+1 may be obtained by reducing the contact or
near-contact heater power by a predetermined amount known to induce
a desired negative displacement. In other cases, the heater power
may be gradually reduced while actively measuring clearance until
the desired clearance is reached. In such a case, a heater power
that results in the desired clearance is saved as the new optimum
heater power LP.sub.N+1.
[0030] At block 314, the current optimum heater and laser power are
compared are both compared (e.g., by subtraction, comparison of
discrete DAC values, etc.) to previous optimum heater and laser
power, and if their difference is greater than a threshold amount
(which may be zero), then the counter N is incremented at block 316
and the next iteration begins at block 306. If the difference is
less or equal to the threshold, then the last used heater and laser
power are prepared for use (e.g., stored in memory) for subsequent
use in writing data, as indicated at block 318.
[0031] The procedure shown in FIG. 3 can be performed for each
read/write head in a device, and performed over different zones of
a recording medium. In a controlled test environment (e.g.,
qualification testing) other factors can also be varied while
performing the tests, such as ambient temperature. For each
combination of variables, an optimum heater and laser power may be
determined, either through direct lookup of test results performed
at the corresponding conditions, and/or via extrapolation of
results performed at a subset of the conditions. The procedure may
be performed during manufacture of the device and/or during
operational use.
[0032] In FIG. 4, a graph illustrates results of the example
procedure according to an example embodiment performed on a number
of HAMR hard drive read/write heads. The number of iterations is on
the x-axis and the clearance is on the y-axis. It has been found
that, in most cases, the system can reach the optimal combination
of laser power and heater power in as few as three iterations. The
chart shows that initially there is a large distribution in writer
clearance for each head but after three iterations of the method
described above, nearly all head are writing at a fixed target
clearance indicated by line 400. In these tests SNR was used to
determine the optimum laser power during the laser power sweep part
of the test.
[0033] In FIG. 5, a block diagram illustrates a data storage system
according to an example embodiment. A data storage apparatus 500
includes logic circuitry 502 used to read data from and write data
to one or more magnetic disks 510. The magnetic disks 510 are
configured as a heat-assisted magnetic recording medium. The logic
circuitry 502 includes one or more controllers 504 that perform
operations associated with storing and retrieving data from the
disks 510. The operations include processing read and write
commands that originate from a host device 506. The host device 506
may include any electronic device that can be communicatively
coupled to store and retrieve data from a data storage device,
e.g., a computer, peripheral bus card, etc.
[0034] The controller 504 is coupled to a read/write channel 508
that processes data read from and written to the magnetic disk 510.
The read/write channel 508 generally converts data between the
digital signals processed by the controller 504 and the analog
signals conducted through one or more read/write heads 512 (also
referred to as a recording head). The read write heads 512 are
positioned over the magnetic disk 510 via a servo motor 514 (e.g.,
voice coil motor) that moves one or more arms 516 to which the
read/write heads 512 are mounted.
[0035] During read and write operations, a heater control circuit
518 sends power to one or more heaters of the read/write head 512.
The heater control circuit 518 may include a DAC, preamplifier,
filters, etc., that control and condition signals send to the
reader heaters, which are used to adjust dynamic HMS between the
read/write head 512 and disk 510. The controller 504 may receive
feedback signals (not shown) that assist in controlling the heater,
such as temperature readings from a head-mounted thermal sensor, AE
detection, etc.
[0036] During write operations, a laser control circuit 520 sends
power to one or more lasers (or similar thermal energy producing
devices) of the read/write head 512. The laser control circuit 520
may include a DAC, preamplifier, filters, etc., that control and
condition signals send to the lasers, which are used energize a
near-field transducer that creates a hotspot on the disk 510 during
recording. The controller 504 may receive feedback signals (not
shown) that assist in controlling the laser, such as intensity
readings from a head-mounted photodiode, etc. The laser control
circuit 520 may adjust laser power to different levels during
writing. For example, when traversing servo marks on the disk 510,
the laser may be kept at a bias current that keeps the laser active
but does not cause the laser to emit enough energy to heat the disk
510 to the Curie temperature, thereby preventing corruption of data
stored in the servo marks.
[0037] The controller 504 may access a persistent storage to access
instructions and data used in operating the apparatus 500. The
persistent storage may include any combination of the primary
storage medium (the disk 510 in this case) and local non-volatile
solids-state data storage media, such as flash memory. One example
of instructions and data that may be stored is represented by
calibration module 522.
[0038] The calibration module 522 includes instructions that cause
the controller 504 to perform a calibration procedure. The
procedure involves setting (e.g., via the heater controller 518) a
heater power of the heat-assisted magnetic recording head 512 to an
initial power. Thereafter, a plurality of iterations are performed
on the data storage apparatus 500. The iterations involve varying a
laser power of the recording head 512 while writing data to at
least one track of a recording medium 510 at the heater power. An
optimum laser power is determined based on reading the data. During
each iteration, an additional heater power is applied to cause a
head-medium contact clearance at the optimum laser power. A heater
power for the next iteration is set based on an offset from the
additional heater power.
[0039] During the iterations, a first difference in the heater
power is determined between two subsequent iterations. A second
difference between the optimum laser power between two subsequent
iterations also determined. If the first difference is below a
first threshold and the second difference is below a second
threshold, the iterations are stopped. The heater power and the
optimum laser power for the last iteration are used as an
operational heater power and an operational laser power for the
heat-assisted magnetic recording head 512. The values of the
operational heater power and the operational laser power may be
stored on the apparatus 500, e.g., in non-volatile data storage. It
will be understood that some or all of the instructions that cause
the controller to perform the calibration procedure may be provided
from an external source. For example, the host 506 may be
configured as a testing device that directs the calibration
procedure as part of qualification testing.
[0040] In reference now to FIG. 6, a flowchart illustrates a method
according to an example embodiment. The method involves setting 600
a heater power of a heat-assisted magnetic recording head to an
initial power to induce an initial head-medium clearance. A
plurality of iterations are performed, as indicated by block 601.
For each iteration, a laser power of the recording head is varied
602 while writing data to at least one track of a recording medium
at the initial head-medium clearance. An optimum laser power is
determined 603 based on reading the data. The optimum laser power
may be determined 603 based on any combination of: a
signal-to-noise ratio of the at least one track; a bit-error rate
of the at least one track; a bit-error rate of multiple adjacent
tracks; a servo error rate of the at least one track a servo
variable gain amplifier gain while reading back the at least one
track; a width of the at least one track; and an amplitude of the
at least one track.
[0041] An additional heater power is applied 604 to approach or
cause a head-medium contact at the optimum laser power. In a case
where the additional heater power causes the head-medium contact at
the optimum laser power, the detecting the head-medium contact may
be based on acoustic emissions resulting from the head-medium
contact. In a case where the wherein the additional heater power
causes and approach to the head-medium contact (but does not cause
contact) at the optimum laser power, the additional heater power
may be based on a head-to-medium clearance that is detected before
the head-medium contact occurs. In either case, the heater power is
set 605 for the next iteration based on an offset from the
additional heater power.
[0042] It is determined 606 whether a first difference in the
heater power between two subsequent iterations is below a first
threshold and also determined 607 whether a second difference in
the optimum laser power between the two subsequent iterations is
below a second threshold. If both determinations 606, 607 are
positive, the iterations stop and the heater power and the optimum
laser power for one of the two subsequent iterations are used 608
respectively as an operational heater power and an operational
laser power for the heat-assisted magnetic recording head.
[0043] The various embodiments described above may be implemented
using circuitry and/or software modules that interact to provide
particular results. One of skill in the computing arts can readily
implement such described functionality, either at a modular level
or as a whole, using knowledge generally known in the art. For
example, the flowcharts illustrated herein may be used to create
computer-readable instructions/code for execution by a processor.
Such instructions may be stored on a non-transitory
computer-readable medium and transferred to the processor for
execution as is known in the art.
[0044] Unless otherwise indicated, all numbers expressing feature
sizes, amounts, and physical properties used in the specification
and claims are to be understood as being modified in all instances
by the term "about." Accordingly, unless indicated to the contrary,
the numerical parameters set forth in the foregoing specification
and attached claims are approximations that can vary depending upon
the desired properties sought to be obtained by those skilled in
the art utilizing the teachings disclosed herein. The use of
numerical ranges by endpoints includes all numbers within that
range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and
any range within that range.
[0045] The foregoing description of the example embodiments has
been presented for the purposes of illustration and description. It
is not intended to be exhaustive or to limit the embodiments to the
precise form disclosed. Many modifications and variations are
possible in light of the above teaching. Any or all features of the
disclosed embodiments can be applied individually or in any
combination are not meant to be limiting, but purely illustrative.
It is intended that the scope of the invention be limited not with
this detailed description, but rather determined by the claims
appended hereto.
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